The frozen soil deep below the Siberian tundra has yielded an astonishing discovery: giant viruses that are still infectious though they've spent the last 30,000 years or so on ice.

The newfound virus, although invisible to the naked eye, is the biggest on Earth -- larger even than many bacteria and some regular cells -- and may be a new branch of a giant-virus family tree.

It poses no danger to people, infecting only one-celled creatures called amoebas. But the scientists who identified the new super-virus say their work shows that humans who disturb permafrost, or frozen soil, may encounter viruses that aren't as benign. It's a growing possibility as climate change warms the far north.

The perfmafrost may harbor viruses harmful to humans or animals, and "at the moment, there's no reason to believe they (aren't) also resistant for a long time under the same conditions," says study author Jean-Michel Claverie of Aix-Marseille University in France. Now that the loss of sea ice has made mineral-rich north Siberia more accessible, "there will be people where there were no people before, and those people are going to manipulate and disturb those (soil) layers that have not been disturbed for a million years."

The unlikely source of the new microbe is a layer of permafrost 100 feet beneath the treeless tundra in northeastern Russia. The soil has been frozen for thousands of years, shielded from the oxygen and light that degrade living material.

Hoping the permafrost might have made a safe haven for prehistoric viruses, the researchers added some excavated material to colonies of amoebas, tiny, blobby organisms that live in water and soil. Some of the amoebas burst and died, suggesting they'd been infected by a virus. That virus was a fat, elongated giant, according to the new study. The scientists christened it Pithovirus, from the Greek word "pithos," a large ceramic storage barrel used in ancient times, they say in this week's Proceedings of the National Academy of Sciences.

Pithovirus is bigger than any other giant virus and 10 to 20 times larger than ordinary viruses but still much smaller than a grain of sand.

As for how giant viruses evolved, Claverie says, "frankly, at the moment, your guess is as good as ours."